1. Field of the Invention
The present invention relates to a reflective mask blank for EUV (Extreme Ultra Violet) lithography (hereinbelow, referred to as “EUV mask blank” in this Description) to be used for semiconductor manufacturing or the like, and a substrate with a conductive film to be used for production of the mask blank.
2. Discussion of Background
In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for writing, on a Si substrate or the like, a fine pattern, which is required for writing an integrated circuit comprising such a fine pattern. However, the conventional exposure techniques using light exposure have been close to the exposure limit while semiconductor devices have had finer patterns at an accelerated pace. In the case of light exposure, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, has been considered as being promising as the exposure technique for 45 nm or below. In this Description, it should be noted that the phrase “EUV light” means a ray having a wavelength in a soft X ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
It is impossible to use EUV light in conventional dioptric systems as in photolithography using visible light or ultraviolet light since EUV light is apt to be absorbed by any substances and since the refractive index is close to 1 when EUV light is absorbed. For this reason, a catoptric system, i.e., a combination of a reflective photomask and a mirror, is employed in EUV light lithography.
A mask blank is a stacked member for fabrication of a photomask, which has not been patterned yet. When a mask blank is used for a reflective photomask, the mask blank has a structure wherein a substrate made of glass or the like has a reflective layer for reflecting EUV light and an absorbing layer for absorbing EUV light, formed thereon in this order. The reflective layer normally comprises a reflective multilayer film, which comprises high-refractive layers and low-refractive layers alternately stacked to increase a light reflectance when irradiating a film surface with a ray, more specifically when irradiating a film surface with EUV light. The absorbing layer comprises a material having a high absorption coefficient in connection with EUV light, specifically, for example, a material containing Cr or Ta as the main component.
The reflective multilayer film and the absorbing layer are deposited by ion beam sputtering or magnetron sputtering. When the reflective multilayer film and the absorbing layer are deposited, the substrate is supported by a supporting means. Although there are a mechanical chuck and an electrostatic chuck as the supporting means, an electrostatic chuck is preferably used in view of the generation of dust. Further, an electrostatic chuck is used as a means to support the substrate also at the time of the mask patterning process or during mask handling at the time of light exposure. However, when the substrate has a low dielectric constant and a low conductivity as in a glass substrate, there is a risk that dielectric breakdown is caused since a high voltage is required to be applied in order to obtain a chucking force at the same level as, e.g., a silicon wafer.
In order to solve such a problem, Patent Document 1 discloses a mask substrate having back coating (conductive film) of a material other than conventional Cr, for example, a substance having a higher dielectric constant and a higher conductivity than those of a glass substrate, such as Si, Mo, chromium oxynitride (CrON) or TaSi, as a layer to accelerate electrostatic chucking of the substrate.
However, the mask substrate disclosed in Patent Document 1 has had such drawbacks that film peeling between the glass substrate and the CrON film is likely to occur, thus generating particles during deposition of a reflective multilayer film and an absorbing layer, since the adhesion of the CrON film to the glass substrate is weak. Particularly, in the vicinity of the boundary between the electrostatic chuck and the CrON film, film peeling is likely to occur caused by force applied to the vicinity of the boundary between the electrostatic chuck and the CrON film due to rotation of the substrate.
Further, the mask substrate disclosed in Patent Document 1 has a conductive film formed on the whole of one face including the chamfered face and the side face of the substrate, and accordingly, film peeling is likely to occur especially on the chamfered face and the side face of the substrate due to warpage of the substrate at the time of electrostatic chucking or the like in such a state that the film adhesion is particularly weak by formation of the conductive film on the chamfered face and the side face at a slant.
Further, in the mask substrate disclosed in Patent Document 1, oxygen (O) is contained in the surface of the CrON conductive film, and accordingly abnormal discharge may occur at the time of depositing the reflective multilayer film or the absorbing film depending upon the deposition conditions.
If such film peeling of a conductive film occurs at the time of electrostatic chucking (at the time of film deposition) or if particles are generated by abnormal discharge during film deposition, it is impossible to obtain a high quality product (a substrate with a reflective multilayer film, a reflective mask blank for exposure or a reflective mask for exposure) because of an increase in the formation of defects in the product. In a case where a pattern is written by using a conventional transmission mask for exposure, even when a defect of irregularities is caused on a mask surface, the presence of such a defect seldom have a significant adverse effect since exposure light has a relatively long wavelength, which is in an ultraviolet range (about 157 to about 248 nm). Accordingly, no special recognition has been given as a problem to be solved, to the generation of particles during film deposition. However, when light having a short wavelength such as EUV light is used as exposure light, it is impossible to ignore the generation of particles since even the presence of the defect of fine irregularities has a significant adverse effect to a printed image.
To solve the above problem, Patent Document 2 proposes a substrate with a reflective multilayer film which suppresses, at the time of electrostatic chucking of a substrate with a conductive film, film peeling of the conductive film and generation of particles due to abnormal discharge, a high quality reflective mask blank for exposure with reduced surface defects due to particles, and a high quality reflective mask for exposure without pattern defects due to particles.
The substrate with a reflective multilayer film disclosed in Patent Document 2 has a conductive film formed in a region excluding at least the peripheral portion of the substrate, in order to prevent generation of particles due to film peeling of the conductive film at the substrate peripheral portion. Further, in the substrate with a reflective multilayer film disclosed in Patent Document 2, the surface of the conductive film to be brought into contact with the electrostatic chuck at the time of electrostatic chucking comprises a metal nitride film containing substantially no oxygen (O) in order to prevent generation of abnormal discharge at the time of film deposition of the reflective multilayer film and the absorbing film. Further, in the substrate with a reflective multilayer film disclosed in Patent Document 2, the composition of the material forming the conductive film is changed in the conductive film thickness direction, in order to improve both the adhesion of the conductive film to the substrate and the adhesion between the electrostatic chuck and the substrate and to prevent generation of particles due to film peeling of the conductive film or generation of particles due to rubbing between the electrostatic chuck and the substrate caused by insufficient adhesion between the electrostatic chuck and the substrate. Namely, the conductive film has such a structure that it contains nitrogen (N) on the substrate side, and it contains at least one of oxygen (O) and carbon (C) on the surface side of the conductive film.
Namely, in the substrate with a reflective multilayer film disclosed in Patent Document 2, generation of particles at the time of film deposition is prevented by the following (1) to (4).
(1) Film peeling of the conductive film at the substrate peripheral portion is prevented by not forming the conductive film on the substrate peripheral portion.
(2) Generation of particles at the time of film deposition is prevented by preventing generation of abnormal discharge at the time of film deposition.
(3) Generation of particles due to film peeling of the conductive film at the time of film deposition is prevented by improving adhesion of the conductive film to the substrate.
(4) Generation of particles due to rubbing between the electrostatic chuck and the substrate caused by insufficient adhesion between the electrostatic chuck and the substrate is prevented by improving the adhesion between the electrostatic chuck and the substrate.
Patent Document 1: JP-A-2003-501823
Patent Document 2: JP-A-2005-210093